To evaluate the expression of estrogen receptors (ER) a and b, and activity of alkaline phosphatase during differentiation of primary osteoblast cells (hOB) from aged postmenopausal women and human osteosarcoma cell lines (HOS, MG63).
Materials and Methods
Osteoblast cultures were prepared from the upper femur of postmenopausal patients (age, 60-74 years) and HOS. At the indicated times (days 5, 10, 15, 20, and 25), alkaline phosphatase activity and expression of ERa and ERb mRNA were evaluated.
In both cultures of primary hOB and HOS, alkaline phosphatase activity decreased at the osteoblast proliferation stage, whereas it subsequently increased at the matrix maturation stage. ERb mRNA was strongly expressed in HOS on day 15 and remained at high levels of transcription through to day 25 (matrix maturation phase), whereas ERa mRNA was barely detectable during osteoblast differentiation. In hOB, transcription of ERa mRNA was much stronger than that of ERb mRNA.
The presence of ERa and ERb mRNA in osteoblasts supports the involvement of estrogen in human bone formation. The developmental expression of alkaline phosphatase was not correlated to ER mRNA expression during osteoblast differentiation. ER isoforms may have different functions or interact with each other during osteoblast differentiation. Since the expression of ER isoforms is different between postmenopausal women and osteosarcoma cell lines, characteristics of osteosarcoma cell lines may not be suitable as a model for the evaluation of estrogen effects on postmenopausal osteoporosis.